The overall goal of this proposal is to define the molecular interactions between hematopoietic stem/progenitor cells (HSPC) and their microenvironment using two distinct in vivo model systems. One is murine and disease focused. The other is zebrafish and focused on the normal. Both will employ high resolution imaging. The murine system takes advantage of a unique finding that modifying a single gene associated with RNA processing (Dicer1) in a single subset of mesenchymal cells can dramatically alter hematopoiesis, creating a complex syndrome resembling human myelodysplasia. Defining the basis for the alteration in HSPC function will reveal molecular regulators at the niche-HSPC interface that we propose to then test in the zebrafish system, developing methods to enhance the use of that model in studies of the niche. In parallel, moderate through-put screening technology has been developed for the zebrafish that will be used to identify modifiers of HSPC engraftment of the niche. These will then be further assessed in murine systems establishing convergence or divergence of the two models. The two approaches will be conducted concurrently with data exchange between them enabling testing of information from each system in the other. Through this interactive exchange of two distinct systems, we anticipate a rapid identification and verification of mediators and potential chemical perturbogens of hematopoietic-microenvironment interactions that can then be considered for clinical application. Our aims are to: 1. Define the molecular basis for complex dysfunctional hematopoiesis with osteoprogenitor specific deletion of Dicer1 in the mouse. 2. Identify modifiers of HSPC-niche interactions in the zebrafish. 3. Determine the correspondence between zebrafish and mouse hematopoietic stem cell interactions with the niche Successful accomplishment of these aims will yield insight into normal and dysfunctional stem cell interactions with the niche and provide potential means of altering them. Further, it will define the ability to combine the strengths of the mouse to model disease with the strengths of the zebrafish to conduct broad screens as a means of accelerating discovery in hematopoiesis research. PUBLIC HEALTH RELEVANCE: The overall goal of this proposal is to define the basis for a mouse model of the human blood disease, myelodysplasia and to identify compounds that might positively affect the bone marrow. We will use a unique combination of mouse and zebrafish model systems examined by molecular, chemical and imaging techniques.